Process and apparatus for hot catalyst stripping in a bubbling bed catalyst regenerator

ABSTRACT

A process and apparatus for achieving multistage, hot catalyst stripping of spent FCC catalyst in a bubbling bed regenerator having a stripper mounted over the regenerator and a stripped catalyst standpipe within the regenerator. A secondary or hot catalyst stripper is placed under the primary stripper and within the existing regenerator vessel. Spent catalyst from the primary stripper is heated in the secondary stripper by at least one of immersion in the bubbling dense bed of hot regenerated catalyst, addition of hot regenerated catalyst recovered from the discharged into the coke combustor and regenerated in a turbulent or fast fluidized bed, and discharged up into a dilute phase transport riser which preferably encompasses, and is in a countercurrent heat exchange relationship with, the spent catalyst standpipe. Regenerated catalyst is discharged from the dilute phase transport riser, and collected in the bubbling dense bed surrounding the coke combustor. Catalyst may be recycled from the dense bed to the coke combustor for direct contact heat exchange. Catalyst coolers may be used on catalyst recycle lines to the coke combustor, or on the line returning regenerated catalyst to the cracking reactor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process and apparatus for stripping andregenerating fluidized catalytic cracking catalyst.

2. Description of Related Art

In the fluidized catalytic cracking (FCC) process, catalyst, having aparticle size and color resembling table salt and pepper, circulatesbetween a cracking reactor and a catalyst regenerator. In the reactor,hydrocarbon feed contacts a source of hot, regenerated catalyst. The hotcatalyst vaporizes and cracks the feed at 425C.-600C., usually460C.-560C. The cracking reaction deposits carbonaceous hydrocarbons orcoke on the catalyst, thereby deactivating the catalyst. The crackedproducts are separated from the coked catalyst. The coked catalyst isstripped of volatiles, usually with steam, in a catalyst stripper andthe stripped catalyst is then regenerated. The catalyst regeneratorburns coke from the catalyst with oxygen containing gas, usually air.Decoking restores catalyst activity and simultaneously heats thecatalyst to, e.g., 500C.-900C., usually 600C.-750C. This heated catalystis recycled to the cracking reactor to crack more fresh feed. Flue gasformed by burning coke in the regenerator may be treated for removal ofparticulates and for conversion of carbon monoxide, after which the fluegas is normally discharged into the atmosphere.

Catalytic cracking has undergone progressive development since the 40s.The trend of development of the fluid catalytic cracking (FCC) processhas been to all riser cracking and use of zeolite catalysts. A goodoverview of the importance of the FCC process, and its continuousadvancement, is reported in Fluid Catalytic Cracking Report, Amos A.Avidan, Michael Edwards and Hartley Owen, as reported in the Jan. 8,1990 edition of the Oil & Gas Journal.

Modern catalytic cracking units use active zeolite catalyst to crack theheavy hydrocarbon feed to lighter, more valuable products. Instead ofdense bed cracking, with a hydrocarbon residence time of 20-60 seconds,much less contact time is needed. The desired conversion of feed can nowbe achieved in much less time, and more selectively, in a dilute phase,riser reactor.

Although reactor residence time has continued to decrease, the height ofthe reactors has not. Although the overall size and height of much ofthe hardware associated with the FCC unit has decreased, the use of allriser reactors has resulted in catalyst and cracked product beingdischarged from the riser reactor at a fairly high elevation. Thiselevation makes it easy for a designer to transport spent catalyst fromthe riser outlet, to a catalyst stripper at a lower elevation, to aregenerator at a still lower elevation.

The need for a somewhat vertical design, to accommodate the great heightof the riser reactor, and the need to have a unit which is compact,efficient, and has a small "footprint", has caused considerableevolution in the design of FCC units, which evolution is reported to alimited extent in the Jan. 8, 1990 Oil & Gas Journal article. Onemodern, compact FCC design is the Kellogg Ultra Orthoflow converter,Model F, which is shown in FIG. 1 of this patent application, and alsoshown as FIG. 17 of the Jan. 8, 1990 Oil & Gas Journal article discussedabove. The compact nature of the design, and the use of a catalyststripper which is contiguous with and supported by the catalystregenerator, makes it difficult to expand or modify such units. Thecatalyst stripper design is basically a good one, which achieves someefficiencies because of its location directly over the bubbling bedregenerator. The stripper can be generously sized, does not have to fitaround the riser reactor as in many other units, and the stripper iswarmed slightly by its close proximity to the regenerator, which willimprove its efficiency slightly.

Although such a unit works well in practice, the stripping of spentcatalyst is never as complete as desired by the refiner. In addition,FCC units are being pushed to accept poorer feeds, particularly feedscontaining large amounts of resid. These growing demands placed on FCCunits and exacerbated four existing problem areas in the regenerator,namely problems with sulfur, steam, temperature and NOx. These problemswill each be reviewed in more detail below.

SULFUR

Much of the sulfur in the feed ends up as SOx in the regenerator fluegas. Higher sulfur levels in the feed, combined with a more completeregeneration of the catalyst in the regenerator increases the amount ofSOx in the regenerator flue gas. Some attempts have been made tominimize the amount of SOx discharged to the atmosphere through the fluegas by including catalyst additives or agents to react with the SOx inthe flue gas. These agents pass with the regenerated catalyst back tothe FCC reactor where the reducing atmosphere releases the sulfurcompounds as H2S. Suitable agents are described in U.S. Pat. Nos.4,071,436 and 3,834,031. Use of cerium oxide agent for this purpose isshown in U.S. Pat. No. 4,001,375.

Unfortunately, the conditions in most FCC regenerators are not the bestfor SOx adsorption. The high temperatures in modern FCC regenerators (upto 870° C. (1600° F.)) impair SOx adsorption. One way to minimize SOx influe gas is to pass catalyst from the FCC reactor to a long residencetime steam stripper, as disclosed in U.S. Pat. No. 4,481,103 to Krambecket al which is incorporated by reference. This process preferably steamstrips spent catalyst at 500-550° C. (932 to 1022° F.), which isbeneficial but not sufficient to remove some undesirable sulfur- orhydrogen-containing components.

STEAM

Steam is always present in FCC regenerators although it is known tocause catalyst deactivation. Steam is not intentionally added, but isinvariably present, usually as adsorbed or entrained steam from steamstripping or catalyst or as water of combustion formed in theregenerator.

Poor stripping leads to a double dose of steam in the regenerator, firstfrom the adsorbed or entrained steam and second from hydrocarbons lefton the catalyst due to poor catalyst stripping. Catalyst passing from anFCC stripper to an FCC regenerator contains hydrogen-containingcomponents, such as coke or unstripped hydrocarbons adhering thereto.This hydrogen burns in the regenerator to form water and causehydrothermal degradation.

Steaming of catalyst becomes more of a problem as regenerators gethotter. Higher temperatures greatly accelerate the deactivating effectsof steam.

TEMPERATURE

Regenerators are operating at higher and higher temperatures. This isbecause most FCC units are heat balanced, that is, the endothermic heatof the cracking reaction is supplied by burning the coke deposited onthe catalyst. With heavier feeds, more coke is deposited on the catalystthan is needed for the cracking reaction. The regenerator gets hotter,and the extra heat is rejected as high temperature flue gas. Manyrefiners severely limit the amount of resid or similar high CCR feeds tothat amount which can be tolerated by the unit. High temperatures are aproblem for the metallurgy of many units, but more importantly, are aproblem for the catalyst. In the regenerator, the burning of coke andunstripped hydrocarbons leads to much higher surface temperatures on thecatalyst than the measured dense bed or dilute phase temperature. Thisis discussed by Occelli et al in Dual-Function Cracking CatalystMixtures, Ch. 12, Fluid Catalytic Cracking, ACS Symposium Series 375,American Chemical Society, Washington, D.C., 1988.

Some regenerator temperature control is possible by adjusting the CO/CO2ratio produced in the regenerator. Burning coke partially to CO producesless heat than complete combustion to CO2. However, in some cases, thiscontrol is insufficient, and also leads to increased CO emissions, whichcan be a problem unless a CO boiler is present.

U.S. Pat. No. 4,353,812 to Lomas et al, which is incorporated byreference, discloses cooling catalyst from a regenerator by passing itthrough the shell side of a heat-exchanger with a cooling medium throughthe tube side. The cooled catalyst is recycled to the regeneration zone.The Kellogg H.O.C. regenerator has a catalyst cooler connected to thedense bed of the regenerator. These approaches remove heat from theregenerator, but will not prevent poorly, or even well, strippedcatalyst from experiencing very high surface or localized temperaturesin the regenerator.

The prior art also used dense or dilute phase regenerated fluid catalystheat removal zones or heat-exchangers that are remote from, and externalto, the regenerator vessel to cool hot regenerated catalyst for returnto the regenerator. Examples of such processes are found in U.S. Pat.Nos. 2,970,117 to Harper; 2,873,175 to Owens; 2,862,798 to McKinney;2,596,748 to Watson et al; 2,515,156 to Jahnig et al; 2,492,948 toBerger; and 2,506,123 to Watson.

NOX

Burning of nitrogenous compounds in FCC regenerators has long led tocreation of minor amounts of NOx, some of which were emitted with theregenerator flue gas. Usually these emissions were not much of a problembecause of relatively low temperature, a relatively reducing atmospherefrom partial combustion of CO and the absence of catalytic metals likePt in the regenerator which increase NOx production.

Many FCC units now operate at higher temperatures, with a more oxidizingatmosphere, and use CO combustion promoters such as Pt. These changes inregenerator operation reduce CO emissions, but usually increase nitrogenoxides (NOx) in the regenerator flue gas. It is difficult in a catalystregenerator to completely burn coke and CO in the regenerator withoutincreasing the NOx content of the regenerator flue gas, so NOx emissionsare now frequently a problem. These problems are more severe in bubblingbed regenerators, because of relatively poor catalyst circulation (largestagnant regions in the dense bed) and the presence of large bubbles ofregeneration gas which leads to localized high concentrations of oxygen,which increases NOx emissions.

Recent catalyst patents include U.S. Pat. No. 4,300,997 and its divisionU.S. Pat. No. 4,350,615, both directed to the use of Pd-Ru CO-combustionpromoter. The bi-metallic CO combustion promoter is reported to do anadequate job of converting CO to CO2, while minimizing the formation ofNOx.

U.S. Pat. No. 4,199,435 suggests steam treating conventional metallic COcombustion promoter to decrease NOx formation without impairing too muchthe CO combustion activity of the promoter.

Process modifications are suggested in U.S. Pat. No. 4,413,573 and U.S.Pat. No. 4,325,833 directed to two-and three-stage FCC regenerators,which reduce NOx emissions.

U.S. Pat. No. 4,313,848 teaches countercurrent regeneration of spent FCCcatalyst, without backmixing, to minimize NOx emissions.

While such process modifications may be useful for new construction theycan not be easily added to existing units, especially not to compactregenerator/stripper designs such as the Kellogg H.O.C. regenerator.

U.S. Pat. No. 4,309,309 teaches the addition of a vaporizable fuel tothe upper portion of a FCC regenerator to minimize NOx emissions. Oxidesof nitrogen formed in the lower portion of the regenerator are reducedin the reducing atmosphere generated by burning fuel in the upperportion of the regenerator.

U.S. Pat. No. 4,235,704 suggests that too much CO combustion promotercauses NOx formation, and calls for monitoring the NOx content of theflue gases, and adjusting the concentration of CO combustion promoter inthe regenerator based on the amount of NOx in the flue gas.

The approach taken in U.S. Pat. No. 4,542,114 is to minimize the volumeof flue gas by using oxygen rather than air in the FCC regenerator, withconsequent reduction in the amount of flue gas produced.

All the catalyst and process patents discussed above, directed toreducing NOx emissions, from U.S. Pat. No. 4,300,997 to U.S. Pat. No.4,542,114, are incorporated herein by reference.

The reduction in NOx emissions achieved by the above approaches helpssome but still may fail to meet the ever more stringent NOx emissionslimits set by local governing bodies. Much of the NOx formed is not theresult of combustion of N2 within the FCC regenerator, but rathercombustion of nitrogen-containing compounds in the coke entering the FCCregenerator.

Unfortunately, the trend to heavier feeds usually means that the amountof nitrogen compounds on the coke will increase so NOx emissions willincrease. Higher regenerator temperatures also tend to increase NOxemissions. It would be beneficial, in existing refineries, to have a wayto reduce NOx emissions so that heavier feeds, and environmentalconcerns, can be accommodated.

We realized that a better catalyst stripper design is needed. A betterstripper would attack most of the problems in the regenerator at theirsource, namely poor stripping. Better stripping would permit increasedrecovery of valuable, strippable hydrocarbons and remove more hydrogenfrom spent catalyst to minimize hydrothermal degradation in theregenerator. It would also remove more sulfur-containing compounds fromspent catalyst prior to regeneration to minimize SOx in the regeneratorflue gas and would help reduce regenerator temperature by reducing theamount of material burned in it. The problems were obvious, but asolution to these problems, which could be incorporated into existingFCC regenerators, especially compact designs, was not.

We reviewed the work that others had done on improving stripping, andfound nothing directly applicable to the special problems of betterstripping in FCC units where the stripper was so closely associatedwith, and supported by, a bubbling dense bed regenerator. Theimprovements in stripping in FCC units where the stripper was remotefrom the regenerator were not directly applicable. Thus we could notreadily use the hot stripper design of U.S. Pat. No. 4,820,404 (Owen),which is easiest to implement in units where the regenerator is at ahigher elevation that the catalyst stripper. For similar reasons wecould not use the multi-stage hot strippers of U.S. Pat. No. 4,789,458(Haddad, Owen, Schatz).

We discovered a way to achieve high temperature stripping of coked FCCcatalyst which could be readily retrofitted into strippers operatingabove and supported by bubbling dense bed regenerators. We discovered away to make the vices of the existing design, it compactness andproximity to the bubbling bed regenerator, virtues which allowed us toachieve an unexpectedly effective hot stripping design. We found a wayto improve stripping, increase the yield of valuable liquid product,reduce the load placed on the catalyst regenerator, minimize SOx and NOxemissions and permit the unit to process more difficult feeds.Regenerator temperatures can be reduced somewhat, and the hydrothermaldeactivation of catalyst in the regenerator reduced. All this could beaccomplished generally within the confines of existing equipment, andwith significantly less regenerated catalyst circulation to the stripper(for direct contact heat exchange) than would be expected based o otherhot stripper designs.

BRIEF SUMMARY OF THE INVENTION

Accordingly, the present invention provides a fluidized catalyticcracking process wherein a heavy hydrocarbon feed comprisinghydrocarbons having a boiling point above about 650 F. is catalyticallycracked to lighter products comprising the steps of catalyticallycracking said feed in a catalytic cracking zone operating at catalyticcracking conditions by mixing, in the base of a riser reactor, a heavycrackable feed with a source of hot regenerated catalytic crackingcatalyst withdrawn from a catalyst regenerator, and cracking said feedin said riser reactor to produce catalytically cracked products andspent catalyst which are discharged from the top of the riser into acatalyst disengaging zone wherein cracked products are separated fromspent catalyst; separating cracked products from spent catalyst in saidcatalyst disengaging zone to produce a cracked product vapor phase whichis recovered as a product and a spent catalyst phase which is dischargedfrom said disengaging zone into a catalyst stripper contiguous with andbeneath said disengaging zone; steam stripping said spent catalyst withstripping steam in said stripping zone to produce a stripper vaporcomprising cracked products and stripping steam which is removed fromsaid stripping zone as a product and a stripped catalyst phasecomprising stripped catalyst having a temperature is discharged into avertical standpipe beneath said stripping zone; discharging strippedcatalyst from said standpipe into a catalyst regeneration zonecontiguous with and beneath said stripping zone; regenerating saidstripped catalyst in a regeneration zone comprises a single dense phasebubbling fluidized bed of catalyst to which an oxygen containingregeneration gas is added and from which hot regenerated catalyst,having a regenerated catalyst temperature above said stripped catalysttemperature, is withdrawn and recycled to said riser reactor,characterized by discharging said stripped catalyst from said catalyststandpipe into a hot stripper means which is at least partially immersedin said bubbling dense bed, and heating said stripped catalyst in saidhot stripper means by indirect heat exchange with hot regeneratedcatalyst in said bubbling dense bed, adding a stripping gas to said hotstripper, and stripping additional cracked products from said strippedcatalyst to produce hot stripped catalyst which is charged to saidcatalyst regeneration zone and hot stripper vapor which is removed as aproduct.

In another embodiment, the present invention provides a process forfluidized catalytic cracking wherein a heavy hydrocarbon feed comprisinghydrocarbons having a boiling point above about 650 F. is catalyticallycracked to lighter products comprising the steps of: catalyticallycracking said feed in a catalytic cracking zone operating at catalyticcracking conditions by mixing, in the base of a riser reactor, a heavycrackable feed with a source of hot regenerated catalytic crackingcatalyst withdrawn from a catalyst regenerator, and cracking said feedin said riser reactor to produce catalytically cracked products andspent catalyst which are discharged from the top of the riser into acatalyst disengaging zone wherein cracked products are separated fromspent catalyst; separating cracked products from spent catalyst in saidcatalyst disengaging zone to produce a cracked product vapor phase whichis recovered as a product and a spent catalyst phase which is dischargedfrom said disengaging zone into a primary catalyst stripper contiguouswith and beneath said disengaging zone; stripping said spent catalystwith stripping gas in said primary stripping zone to produce a strippervapor comprising cracked products and stripping gas which is removedfrom said stripping zone as a product and a stripped catalyst phasecomprising stripped catalyst having a temperature is discharged into avertical standpipe beneath said primary stripping zone; hot strippingcatalyst discharged from from said primary stripper standpipe in a hotcatalyst stripping zone below said primary stripping zone by heatingspent catalyst in said standpipe by indirect, countercurrent heatexchange with a dilute phase mixture of hot regenerated catalyst andflue gas and discharging spent catalyst into a hot stripping zone whichis heated by indirect heat exchange with a fast fluidized or turbulentfluidized bed of catalyst and regeneration gas and stripping the heatedcatalyst in said hot stripping zone with a stripping gas to produce hotstripped catalyst which is discharged from said hot stripping zone andrecovering a hot stripper effluent vapor stream comprising stripping gasand stripped lighter products via a closed conduit means which passesthrough said primary stripper; regenerating hot stripped catalystdischarged from said hot stripping zone in a regeneration zonecomprising a fast fluidized or turbulent fluidized bed of catalystmaintained in a closed coke combustor vessel, which is at leastpartially immersed in said bubbling dense bed, said coke combustorvessel having a base region with a cross sectional area and an upperregion of reduced cross sectional area relative to said base region, byadding an oxygen containing regeneration gas to said coke combustorvessel in an amount sufficient to provide a superficial vapor velocitywhich maintains a majority of the catalyst therein in a state ofturbulent or fast fluidization; transferring said catalyst from saidbase region of said coke combustor to said upper region of said cokecombustor having a reduced cross sectional area, whereby increasing thesuperficial vapor velocity and causing dilute phase catalyst transportin said upper region; discharging at least partially regeneratedcatalyst from said upper region into a dilute phase transport riserconnective with said coke combustor vessel and at least partiallyenclosing said primary stripper catalyst standpipe, said dilute phasetransport riser extending into said dilute phase region within saidregenerator vessel containing said bubbling fluidized bed; anddischarging regenerated catalyst from said dilute phase transport riserand collecting said regenerated catalyst in said bubbling fluidized bedsurrounding said coke combustor and adding to said bubbling fluidizedbed fluffing air in an amount sufficient to maintain a bubblingfluidized bed and to produce a dilute phase region above said bubblingbed comprising fluffing air and regenerated catalyst.

In an apparatus embodiment, the present invention provides an apparatusfor the fluidized catalytic cracking of a heavy feed to lighter morevaluable products comprising: a riser reactor cracking means having abase portion connective with a source of heavy feed and connective witha bubbling dense phase fluidized bed of regenerated catalyst within acatalyst regeneration means; a riser outlet at the top of the riserreactor connective with a catalyst disengaging means adapted to separatea cracked product vapor stream from a spent catalyst stream, anddischarge said spent catalyst into a catalyst stripper means; a primarycatalyst stripping means, located above and supported by said catalystregeneration means, said stripping means adapted to receive spentcatalyst from said disengaging means and contact said spent catalystwith a stripping gas to produce a stripper effluent vapor stream and astripped catalyst stream which is discharged down into a primarystripper catalyst standpipe; and a secondary catalyst stripping means,located beneath said primary catalyst stripping means and at leastpartially within said bubbling dense phase fluidized bed of catalystwithin said regenerator vessel, said secondary catalyst stripping meanshaving an upper portion and a lower portion, said upper portion adaptedto receive stripped catalyst from said primary stripper catalyststandpipe and a lower portion adapted to receive secondary stripping gasand produce secondary stripper vapor which is discharged via at leastone secondary stripping vapor discharge means isolated from and passingthrough said primary stripping means, to discharge stripping vapor fromsaid secondary stripping means to said cracked product vapor stream, andsaid lower portion adapted to discharge catalyst into said regeneratormeans; a catalyst regeneration means adaptive to receive catalystdischarged from said secondary stripping means and maintain saidstripped catalyst as a bubbling dense phase fluidized bed of catalystand regenerate spent catalyst by contact with a source of regenerationgas and produce a regenerated catalyst stream and a dilute phase fluegas stream comprising entrained catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (prior art) is a schematic view of a conventional fluidizedcatalytic cracking unit.

FIG. 2 (invention) is a schematic view of a preferred embodiment of theinvention, showing a stripper heated with catalyst from a regeneratorcyclone.

FIG. 3 (invention) is a schematic view of a multi-stage hot stripper ofthe invention, with a preferred, but optional, fast fluidized bed cokecombustor added to the regenerator.

FIG. 4 (invention) is a schematic view of a multi-stage hot stripper ofthe invention, heated with catalyst from a regenerator cyclone, with anoptional fast fluidized bed coke combustor.

FIG. 5 (invention) is a schematic view of a multi-stage hot stripper ofthe invention, with a preferred method of indirectly heat exchanging thecatalyst in the stripper, and removing stripper vapors.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a simplified schematic view of an FCC unit of the prior art,similar to the Kellogg Ultra Orthoflow converter Model F shown as FIG.17 of Fluid Catalytic Cracking Report, in the Jan. 8, 1990 edition ofOil & Gas Journal.

A heavy feed such as a gas oil, vacuum gas oil is added to riser reactor6 via feed injection nozzles 2. The cracking reaction is completed inthe riser reactor, which takes a 90° turn at the top of the reactor atelbow 10. Spent catalyst and cracked products discharged from the riserreactor pass through riser cyclones 12 which efficiently separate mostof the spent catalyst from cracked product. Cracked product isdischarged into disengager 14, and eventually is removed via uppercyclones 16 and conduit 18 to the fractionator.

Spent catalyst is discharged down from a dipleg of riser cyclones 12into catalyst stripper 8, where one, or preferably 2 or more, stages ofsteam stripping occur, with stripping steam admitted by means not shownin the figure. The stripped hydrocarbons, and stripping steam, pass intodisengager 14 and are removed with cracked products after passagethrough upper cyclones 16.

Stripped catalyst is discharged down via spent catalys standpipe 26 intocatalyst regenerator 24. The flow of catalyst is controlled with spentcatalyst plug valve 36.

Catalyst is regenerated in regenerator 24 by contact with air, added viaair lines and an air grid distributor not shown. A catalyst cooler 28 isprovided so that heat may be removed from the regenerator, if desired.Regenerated catalyst is withdrawn from the regenerator via regeneratedcatalyst plug valve assembly 30 and discharged via lateral 32 into thebase of the riser reactor 6 to contact and crack fresh feed injected viainjectors 2, as previously discussed. Flue gas, and some entrainedcatalyst, are discharged into a dilute phase region in the upper portionof regenerator 24. Entrained catalyst is separated from flue gas inmultiple stages of cyclones 4, and discharged via outlets 8 into plenum20 for discharge to the flare via line 22.

In FIG. 2 (invention) only the changes made to the old regenerator shell24 are shown. Like elements in FIG. 1 and 2 have like numerals.

A multi-stage hot stripper in a standpipe 108 is added to the base ofthe existing catalyst stripper 8. Catalyst from a regenerator cyclone118, discharged via cyclone dipleg 120 heats the catalyst from theprimary stripping zone 8 by direct contact heat exchange. Strippingsteam, or other stripping medium, is added by conventional steamaddition means 122 and 126. The stripped hydrocarbons, and strippinggas, are preferably removed by multiple catalyst withdrawal means,either via side withdrawel means 124, preferably at multiple elevationswithin the stripper, or via one or more central hot stripper vaporrecovery means such as the inverted stacked funnel means 130, definingmultiple annular openings 132 connective with central vapor outlet 134which is connective with the vapor space above the existing stripper 8.

The embodiment shown in FIG. 2 has several important advantages overother hot stripper designs. A significant amount of heating of the hotstripper can be done by indirect heat exchange, because the hot stripperis within the regenerator, and preferably is at least partly immersed inthe regenerator bubbling dense bed, as shown. This is beneficial inseveral ways. The catalyst traffic in the stripper, and in theregenerator is reduced somewhat because of indirect, rather than directcontact, heat exchange. This reduces the size and cost of equipment andslightly reduces catalyst fines or dust lost from the regenerator. Thehot stripper operation is improved, because the concentration of spentcatalyst is higher. Adding hot regenerated catalyst for the purpose ofheating spent catalyst dilutes the spent catalyst, making it harder toremove the last traces of strippable hydrocarbon. Adding hot regeneratedcatalyst also provides additional interstitial space, and to a lesserextent, pore volume, which can entrain stripped hydrocarbons back intothe regenerator.

All stripped product from all stages of stripping can be withdrawntogether, and sent to product fractionation. Some hot strippers of theprior art produced a hot stripper effluent which was not compatible withthe cracked product, i.e., the hot stripper was made hot by partialcombustion, and the hot stripper effluent and cracked product effluentcould not safely be combined. In other hot stripper designs, the hotstripper would be remote from the primary stripper, so elaborate meanshad to be provided to get solid and gasses to and from the hot stripper.In this design, catalyst simply falls by gravity into the hot stripper,while stripped product naturally rises.

The FIG. 2 design also overcomes one of the problems of designing a hotstripper that will function well within the confines of a bubbling densebed regenerator, while being partially above the dense bed. It isbeneficial if at least some hot regenerated catalyst can be added to thestripper, but for reasons of pressure balance, and to permit reliableflow control it is essential that the hot stripper, or standpipe fromit, be well sealed from the bubbling dense bed. The regenerator has ahighly oxidizing atmosphere, while the stripper, and the riser reactoroutlet above the stripper, contain hot hydrocarbons. These oxidizing andreducing atmospheres must be well isolated. There can be no opening fromthe bubbling bed directly into the hot stripper, because this mightallow flow reversal, and because there is no "head" of hot regeneratedcatalyst sufficient to get hot regenerated catalyst into the stripper.The stripper is above the bed, so the tendency of catalyst would be toflow from the stripper into the regenerator, even if pressures in boththe stripper and the regenerator were equal.

The FIG. 2 design permits hot regenerated catalyst to safely be added tothe hot stripper, but does so without dipping into the bubbling densebed for catalyst. Catalyst from a regenerator cyclone, preferably aprimary cyclone 118 as shown in FIG. 2, is discharged from the cyclonedipleg 120 into the hot stripper. This catalyst is usually hotter thancatalyst in the bubbling dense bed, because in most unit some dilutephase afterburning occurs. This catalyst is not only hot, it is high upin the regenerator, and this elevation provides the head need to driveregenerated catalyst into the hot stripper. One or more flow reversalmeans, not shown, may be added for safety, such as a flapper valve, orother hydraulic seal arrangements to ensure that the hot stripper willnot vent into the regenerator, nor the regenerator vent into the hotstripper. The relatively large elevation of the regenerator cyclone, andthe proximity of the hot stripper to the cyclone, permit the use ofrelatively small diameter catalyst flow lines, and allow use ofrelatively large dense beds to seal the dipleg.

The FIG. 2 design is also very tolerant of failure of the hot catalystrecycle line, which is an important consideration of any unit which isexpected to last for years in the erosive environment of a regenerator.If the cyclone 118 discharges too much catalyst, ie., is considered tofail in the "open" flow position, all that will happen is that hotstripper will run a little hotter than normal, and hot regeneratedcatalyst from the cyclone dipleg will be recycle to the bubbling densebed via the hot stripper, rather than directly to the dense bed. If thecyclone 118, or dipleg 120, fails in the "closed" position, i.e., a plugdevelops, then the efficiency of the hot stripping operation will dropoff some because it will not be as hot. The efficiency of cyclone 118will fall to zero, because the dipleg will become full, so all catalystentering will exit with the cyclone exhaust via outlet, which willincrease the duty on the secondary cyclones. Usually the cyclones (bothprimary and secondary) are used in multiples of 4, 6, or 8, and usuallyonly one or two primary cyclones will be needed to supply hotregenerated catalyst to the hot stripper, so loss of efficiency of oneor more cyclones will not be catastrophic, because the other cyclonescan usually handle the overflow.

FIG. 3 shows a preferred embodiment of the invention, with a hotstripper and a preferred, but optional, fast fluidized bed regioncreated in the bubbling bed in the base of the regenerator. The FIG. 3embodiment shows external control of hot regenerated catalyst flow tothe hot stripper.

Conventional stripper 8 discharges stripped catalyst into hot stripper208. Hot stripper 208 is at least partially immersed in a highefficiency regenerator, comprising coke combustor 250 and dilute phasetransport riser 252. In the coke combustor, the air admission rate, andthe cross-sectional area available for flow, and catalyst addition andcatalyst recycle, if any, are adjusted to maintain much or all of thebed in a "fast fluidized condition", characterized by intense agitation,relatively small bubbles, and rapid coke combustion. In terms ofsuperficial vapor velocity and typical FCC catalyst sizes, this meansthe vapor velocity should exceed 3.5 feet per second, preferably is 4-15feet per second, and most preferably is 4-10 feet per second. Thecatalyst density in a majority of the volume in the coke combustor willbe less than 35 pounds/cubic foot, and preferably less than 30pounds/cubic foot, and ideally about 25 pounds/cubic foot, and even lessin the upper regions of the coke combustor, where the diameter of thevessel decreases.

The densities and superficial vapor velocities discussed herein presumethat the unit operates at a pressure where the vast majority of FCCunits operate, namely 25-40 psig. A few might operate at slightly lowerpressures, and a significant minority may operate at somewhat higherpressures, primarily those with power recovery systems. Changes inpressure change the superficial vapor velocity needed to maintain, e.g.,a fast fluidized bed or a bubbling dense bed. It is easy to calculatethe superficial vapor velocity needed to support a given type offluidization, and the bed density expected at those conditions. Ingeneral, an increase in pressure will decrease the superficial vaporvelocity needed to achieve a fast fluidized bed.

The arrangement shown provides a significant amount of indirect,counter-current heat exchange of spent catalyst with regeneratingcatalyst. The first stage of catalyst regeneration takes place in cokecombustor 250, which is operates as a fast fluidized bed. Partiallyregenerated catalyst, and flue gas, are discharged from the fastfluidized bed region and pass as a dilute phase up transport riser 252,which encompasses the lower portion of .hot stripper 208. Partially ortotally regenerated catalyst and flue gas are discharged from thetransport riser via cap or deflector 260, which directs catalyst andflue gas down to the bubbling dense bed. The catalyst tends to continuein a straight line to the bubbling dense bed, while the gas flowssideways, so a measure of catalyst separation is achieved. Catalystdischarged from cap 260 is collected as a bubbling dense bed 265.Additional regeneration gas may be added to dense bed 265, for fluffing,and preferably to obtain an additional stage of regeneration.

Because the base of the stripper, in the FIG. 3 embodiment, is wellbelow the level of bubbling dense bed 265 it is possible to transfercatalyst from bed 265 into the hot stripper via line 220 and slide valve222. Because of the extent of immersion of the hot stripper in the cokecombustor and transport riser, and because of the intense fluidizationwhich occurs in both of these regions, the rate of heat transfer intothe hot stripper via indirect heat exchange can be very high, sorelatively low rates of catalyst recycle via line 220 may be needed.This design will work well even when no catalyst is recycled, providedthat conductive, rather than insulating, refractory materials are usedto line the inside and outside of hot stripper 208.

Although not shown, it is possible, and usually preferred, to provide ameans for recycling some hot regenerated catalyst from bed 265 into thecoke combustor 250. If hot stripping is vigorous enough, or at leastachieves a hot degree of heating coupled with modest additionalstripping, then catalyst recycle from the bubbling dense bed 265 to thecoke combustor 250 may be greatly reduced or eliminated.

A catalyst cooler may also be provided on the regenerated catalystreturn line to the riser reactor, to permit increasing cat:oil ratios inthe unit. A "thimble" cooler, i.e., a vessel connected with and open tosome portion of the regenerator may also be used. In this devicecatalyst flows from a dense bed thimble back into the dense bed by theaction of a fluidizing gas. The thimble operates without catalyst supplyor return lines, and does not require slide valve to control catalystflow, catalyst flow and heat exchange are controlled by the amount offluidizing gas added to the base of the thimble.

In a preferred embodiment of the invention shown in FIG. 3, asignificant amount of combustion air is added to bed 265 both tomaintain fluidization and achieve a significant amount of cokecombustion. Preferably from 5 to 60% of the coke combustion occurs inthe bubbling bed, and most preferably from 10 to 40%. Although bed 265is a typical fluidized bubbling bed, characterized by relatively largestagnant regions, and large bubbles of combustion air which bypass thebed, it is an excellent place to achieve some additional cokecombustion. One of the most significant benefits of coke combustion inbubbling bed 265 is the relatively drier atmosphere. There is a lowersteam partial pressure in the dense bed 265 of the present inventionthan in a conventional dense bed regenerator, such as that shown inFIG. 1. Much of the reduction in steam partial pressure is due to theremoval of water of combustion, and entrained stripping steam, with theflue gas discharged from the coke combustor. By using a fluegas/catalyst separation means on the transport riser outlet, therelatively high steam content flue gas is separated from the catalystwhich is discharged down to form the bubbling fluidized bed. It is alsopossible to greatly reduce the load on the cyclones above the bubblingdense bed, because much less combustion air, and consequently lessentrainment of catalyst into the dilute phase, is needed when only afraction of the coke combustion occurs in the bubbling dense bed. Evenwithout a separation means such as cap 58, the dense bed region 75 ofthe present invention will be drier than the dense bed of theregenerator of FIG. 1 (prior art).

FIG. 4 shows another preferred embodiment of the invention, with a hotstripper and a preferred, but optional, fast fluidized bed regioncreated in the bubbling bed in the base of the regenerator. The FIG. 4embodiment shows internal flow of hot regenerated catalyst flow to thehot stripper, as opposed to the external flow arrangement of FIG. 3.Conventional stripper 8 discharges stripped catalyst into hot stripper308. Catalyst for direct contact heat exchange of spent catalyst in thehot stripper 308 is obtained from a cyclone, preferably a primarycyclone 318, which discharges recovered, hot, regenerated catalyst viadipleg 320 into seal pot 324. This pot is designed to allow apredetermined amount of hot regenerated catalyst to flow via line 328into the hot stripper, while allowing any excess material to simplyoverflow seal pot 324.

This embodiment shows several preferred methods of controlling theamount of hot regenerated catalyst that is allowed to enter hot stripper308. A plurality of steam lines, 330 and 332, are at differentelevations in the hot stripper, under or near the outlet of line 328.When large amounts of fluidizing steam are added via line 330, thedensity of the material above line 330 is greatly reduced, which allowsmore catalyst to flow into the hot stripper. Addition of more steam vialine 332 also reduces density, and can be used to admit more hotregenerated catalyst, and to provide better mixing of spent andregenerated catalyst, and to provide more stripping steam.

Combustion air for the coke combustor is provided via line 301 and airring 303. Air for additional catalyst regeneration, and for fluffing,for the bubbling dense bed is provided via air line 305 and pipe grid307.

FIG. 5 shows a multi-stage hot stripper, with hot stripping section 508below a conventional steam stripper. This embodiment uses a preferredmethod of indirectly heat exchanging the catalyst in the stripper, andremoving stripper vapors.

The hot stripper shown in FIG. 5 addesses the problem of heating thestripper sufficiently to see some improvement in catalyst stripping. Werealized that the conventional stripper, shown in FIG. 1, never achievedanything approaching the stripping temperatures we wanted, primarilybecause the stripper, although partially immersed in the regenerator wasnever efficiently heated by it. Heating was not efficient because therewas not much surface area of the stripper exposed to the regenerator.Assuming that a heat transfer coefficient of around 80 BTU/hr/ft2/F.could be achieved, the stripper design shown in FIG. 1 would never beheated as much as 10 F., and more likely around 2F. of heating would beachieved.

We decided to address the problem in two ways. Increase the surface areaavailable for heat transfer, and also increase the efficiency of thestripping operation by partitioning the hot stripper into multiple zoneswith a very high L/D ratio. A preferred solution is disclosed in FIG. 5.

We took catalyst from a conventional stripper, such as the FIG. 1stripper, and discharged it down into a hot stripper 508 which is atleast partially, and preferrably totally, immersed in the dilute phaseand perhaps even the dense phase region of the bubbling bed regenerator24. By providing a plurality of tubes 510, with inlets 512 at the topfor spent catalyst, and for the discharge of stripper vapor, and outletsat the base of the tube for discharge of stripped catalyst into theregenerator. Stripping gas, preferably steam, is admitted to a lowerportion of each tube 510 to strip and aerate the spent catalyst in thetubes. The aearation should be sufficient to promote vigorous stripping,but not sufficient to blow more than minor amounts of stripped catalystout the tops of the tubes 512.

Using multiple tubes greatly increases the surface area available forheat transfer. In a typical FCC bubbling bed regenerator, use of 20tubes, each 1' in diameter will increase the heating of the stripperthat can be achieved by almost an order of magnitude as compared to theamount of stripper heating that is inherent in the FIG. 1 design.

The removal of stripper vapor from the tubes is facilitated by the useof channel 518 above the tubes 510. This channel isolates the tubes fromthe rush of catalyst discharged by the primary stripper, and providesfor a more orderly addition of spent catalyst to the stripping tubes,and for more orderly withdrawel of stripper vapor. When the tubes 510are radially disposed, as is preferred, channel 518 may be a generallyring shaped baffle above the tubes. The hot stripper 508 should besealed from the regenerator, and this is accomplished by providing sealplate 542 around tubes 510. The tubes 510 should be made of stainlesssteel, or some other equivalent material which is both strong andconductive.

DESCRIPTION OF PREFERRED EMBODIMENTS FCC Feed

Any conventional FCC feed can be used. The process of the presentinvention is especially useful for processing difficult charge stocks,those with high levels of CCR material, exceeding 2, 3, 5 and even 10wt. % CCR.

The feeds may range from the typical, such as petroleum distillates orresidual stocks, either virgin or partially refined, to the atypical,such as coal oils and shale oils. The feed frequently will containrecycled hydrocarbons, such as light and heavy cycle oils which havealready been subjected to cracking.

Preferred feeds are gas oils, vacuum gas oils, atmospheric resids, andvacuum resids, and mixtures thereof. The present invention is veryuseful with heavy feeds having, and with those having a metalscontamination problem. With these feeds, the possibility of reducedburning load in the regenerator, and even more importantly, thepossibility of a dryer regenerator, because of reduced hydrogen contentof coke, will be a significant benefit.

FCC CATALYST

Any commercially available FCC catalyst may be used. The catalyst can be100% amorphous, but preferably includes some zeolite in a porousrefractory matrix such as silica-alumina, clay, or the like. The zeoliteis usually 5-40 wt. % of the catalyst, with the rest being matrix.Conventional zeolites include X and Y zeolites, with ultra stable, orrelatively high silica Y zeolites being preferred. Dealuminized Y (DEALY) and ultrahydrophobic Y (UHP Y) zeolites may be used. The zeolites maybe stabilized with Rare Earths, e.g., 0.1 to 10 wt. % RE.

Relatively high silica zeolite containing catalysts are preferred foruse in the present invention. They withstand the high temperaturesusually associated with complete combustion of CO to CO2 within the FCCregenerator.

The catalyst inventory may also contain one or more additives, eitherpresent as separate additive particles, or mixed in with each particleof the cracking catalyst. Additives can be added to enhance octane(shape selective zeolites, i.e., those having a Constraint Index of1-12, and typified by ZSM-5, and other materials having a similarcrystal structure), adsorb SOX (alumina), remove Ni and V (Mg and Caoxides).

Good additives for removal of SOx are available from several catalystsuppliers, such as Davison's "R" or Katalistiks International, Inc.'s"DeSox."

CO combustion additives are available from most FCC catalyst vendors.

The FCC catalyst composition, per se, forms no part of the presentinvention.

CRACKING REACTOR/REGENERATOR

The FCC reactor and regenerator shell 24, per se, are conventional, andare available from the M.W. Kellogg Company.

The modifications needed to add the hot stripper, and the preferred butoptional combustor pod, or FFB region within, or built partially into,the base of the existing regenerator shell 24 are well within the skillof the art.

FCC REACTOR CONDITIONS

Conventional riser cracking conditions may be used. Typical risercracking reaction conditions include catalyst/oil ratios of 0.5:1 to15:1 and preferably 3:1 to 8:1, and a catalyst contact time of 0.1 to 50seconds, and preferably 0.5 to 5 seconds, and most preferably about 0.75to 2 seconds, and riser top temperatures of 900° to about 1050° F.

HOT STRIPPER CONDITIONS

Conventional hot stripping operating conditions may be used. Typical hotstripper operating conditions include temperatures which are at least20° F. above the temperature in the conventional stripping zone,preferably at least 50° F. above the temperature in the conventionalstripper, and most preferably temperatures in the hot stripper are atleast 100° F. or more hotter.

A stripping gas or medium, preferably steam, is used to augmentstripping. Usually more than 0.1 wt. % steam will be added, preferablyfrom 0.15 to 1 wt. % and most preferably from 0.2 to 0.4 wt. % steam,based on the weight of spent catalyst, is added to the hot strippingzone, in addition to the amount of stripping steam used in theconventional stripper. It is possible, and usually will optimize theoverall operation of the unit, if the total amount of stripping steamused, in both the conventional stripper and the hot stripper, is roughlythe same or increased only slightly. From 10 to 90%, and preferably 20to 60%, of the total amount of stripping medium used is added to the hotstripper. This will reduce the amount of steam added to the primarystripper, and reduce the efficiency of the primary stripper, and shiftthe stripping duty to the hot stripper. In this way, the overallstripping efficiency can be greatly increased, without loading up theprocess lines with steam, and greatly increasing the production of sourwater in downstream units.

The desired heating of spent catalyst in the hot stripper can beachieved by indirect heat exchange, by direct contact heat exchange(recycle of hot regenerated catalyst into the hot stripper) or somecombination of both. Each mode of heating will be briefly reviewed.

DIRECT CONTACT HEAT EXCHANGE

When direct contact heat exchange is practiced, it usually will bepreferred to recycle an amount of regenerated catalyst equal to 10 to500% of the spent catalyst, and preferably from 15 to 150% of the spentcatalyst. The heat balance equations are fairly simple, because the heatcapacity of spent and regenerated catalyst is about the same. A 50/50mix (100% addition of regenerated to spent) of 1000 F. spent and 1350°F. regenerated catalyst will give a mix temperature of abut 1175° F.

INDIRECT HEAT EXCHANGE

Conventional techniques used to calculate the amount of surface neededfor heat exchange may be used. In general, only the basic heat transferequation, Q=UAdT needs to be considered. For a typical FCC regenerator,with a catalyst circulation of 15 tons per minute (30,000 lb/min), it ispossible to estimate fairly closely the amount of heat exchange surfaceneeded to achieve a given temperature rise, say 100° F. The heatcapacity of the FCC catalyst at these conditions is 0.28 BTU/(#-F.), so50.4 MM BTU/Hr of heat must be transferred. A realistic overall heattransfer coefficient is about 80 BTU/Hr-Ft² - F., provided that thetubes are immersed in, or are very near, a dense phase fluidized bed ofcatalyst. There is usually not enough heat present in most dilute phaseregions to permit rapid heat transfer. One exception is the amount ofheat available in a dilute phase transport riser above a coke combustor.Although this stream is, strictly speaking, a dilute phase, it is adilute phase characted by a very high solids content, and a highvelocity. Any heat exchange tube placed in a dilute phase transportriser will exhibit an even higher rate of heat transfer, well in excessof the heat transfer coefficient obtainable in a classical bubbling,dense phase fluidized bed.

With a dT of about 300 F. (assuming 1000 F. for spent catalyst enteringthe hot stripper, and 1100 F. for catalyst leaving the hot stripper, anda 1350 F. average temperature in the regenerator), about 2100 ft² willbe needed. This corresponds to 20 tubes 1.0 feet in diameter, 30 feetlong. This amount of heat exchanger surface can easily be accomodated ina conventional FCC dense bed regenerator.

When the tubes terminate in an additional vessel which is immersed inthe bubling dense bed, there is additional heat transfer into the bottomof the hot stripper via the entire "wetted" area of the immersed hotstripper.

COMBUSTOR POD PROCESS CONDITIONS

Conditions in the optional combustor pod, or FFB region, and in thedilute phase transport riser contiguous with and above it, are verysimilar to those used in conventional High Efficiency Regenerators (HER)now widely used in FCC units. Typical H.E.R. regenerators are shown inU.S. Pat. No. 4,595,567 (Hedrick), U.S. Pat. No. 4,822,761 (Walters,Busch and Zandona) and U.S. Pat. No. 4,820,404 (Owen), which areincorporated herein by reference.

The conditions in the combustor pod comprise a turbulent or fastfluidized bed region in the base, and approach dilute phase flow in theupper regions thereof. These conditions are conventional, what isunconventional is achieving fast fluidized bed catalyst regeneration ina bubbling bed regenerator with a superimposed catalyst stripperdischarging spent catalyst down directly into the regenerator via astandpipe within the dense bed regeneration vessel.

CO COMBUSTION PROMOTER

Use of a CO combustion promoter in the regenerator or combustion zone isnot essential for the practice of the present invention, however, it ispreferred. These materials are well-known.

U.S. Pat. No. 4,072,600 and U.S. Pat. No. 4,235,754, which areincorporated by reference, disclose operation of an FCC regenerator withminute quantities of a CO combustion promoter. From 0.01 to 100 ppm Ptmetal or enough other metal to give the same CO oxidation, may be usedwith good results. Very good results are obtained with as little as 0.1to 10 wt. ppm platinum present on the catalyst in the unit.

DISCUSSION OF HOT STRIPPING BENEFITS

The hot stripper temperature controls the amount of carbon removed fromthe catalyst in the hot stripper. Accordingly, the hot stripper controlsthe amount of carbon (and hydrogen and sulfur) remaining on the catalystto the regenerator. This residual carbon level controls the temperaturerise between the reactor stripper and the regenerator. The hot stripperalso controls the hydrogen content of the spent catalyst sent to theregenerator as a function of residual carbon. Thus, the hot strippercontrols the temperature and amount of hydrothermal deactivation ofcatalyst in the regenerator.

Employing a hot stripper, to remove carbon on the catalyst, rather thana regeneration stage reduces air pollution, and allows all of the carbonmade in the reaction to be burned to CO2, if desired.

The present invention strips catalyst at a temperature higher than theriser exit temperature to separate hydrogen, as molecular hydrogen orhydrocarbons from the coke which adheres to catalyst. This minimizescatalyst steaming, or hydrothermal degradation, which typically occurswhen hydrogen reacts with oxygen in the FCC regenerator to form water.The high temperature stripper (hot stripper) also removes much of thesulfur from coked catalyst as hydrogen sulfide and mercaptans, which areeasy to scrub. In contrast, burning from coked catalyst in a regeneratorproduces SOx in the regenerator flue gas. The high temperature strippingrecovers additional valuable hydrocarbon products to prevent burningthese hydrocarbons in the regenerator.

Another benefit of hot stripping is reduced solids emissions from theregenerator. In many regenerators, solids content of flue gas is roughlyproportional to the solids traffic in the dilute phase of theregenerator. Reducing the solids traffic can reduce the amount of dustand fines that escape the regenerator cyclones. In high efficiencyregenerators, catalyst is recycled from a bubbling dense bed to the cokecombustor, and this catalyst recycle significantly increases catalysttraffic in the regenerator. The hot striper of the present inventionallows heat to be transferred from the regenerator to the catalyst fromthe stripper, without recycling catalyst from the regenerator, or atleast with a reduced amount of catalyst recirculation. This reducedcatalyst load to the coke combustor reduces the amount of catalystdischarged from the coke combustor, and reduces the amount of catalysttraffic in the dilute phase region above the bubbling dense beddownstream of the coke combustor.

We claim:
 1. A fluidized catalytic cracking process wherein a heavyhydrocarbon feed comprising hydrocarbons having a boiling point aboveabout 650° F. is catalytically cracked to lighter products comprisingthe steps of:catalytically cracking said feed in a catalytic crackingzone operating at catalytic cracking conditions by mixing, in the baseof a riser reactor, a heavy crackable feed with a source of hotregenerated catalytic cracking catalyst withdrawn from a catalystregenerator, and cracking said feed in said riser reactor to producecatalytically cracked products and spent catalyst which are dischargedfrom the top of the riser into a catalyst disengaging zone whereincracked products are separated from spent catalyst; separating crackedproducts form spent catalyst in said catalyst disengaging zone toproduce a cracked product vapor phase which is recovered as a productand a spent catalyst phase which is discharged from said disengagingzone into a catalyst stripper contiguous with and beneath saiddisengaging zone; steam stripping said spent catalyst with strippingsteam in said stripping zone to produce a stripper vapor comprisingcracked products and stripping steam which is removed from saidstripping zone as a product and a stripped catalyst phase comprisingstripped catalyst having a temperature is discharged into a verticalstandpipe beneath said stripping zone; discharging stripped catalystfrom said standpipe into a catalyst regeneration zone contiguous withand beneath said stripping zone; regenerating said stripped catalyst ina regeneration zone comprises a single dense phase bubbling fluidizedbed of catalyst to which an oxygen containing regeneration gas is addedand from which hot regenerated catalyst, having a regenerated catalysttemperature above said stripped catalyst temperature, is withdrawn andrecycled to said riser reactor, characterized by: discharging saidstripped catalyst from said catalyst standpipe into a hot stripper meanswhich is at least partially immersed in said bubbling dense bed, andheating said stripped catalyst in said hot stripper means by indirectheat exchange with hot regenerated catalyst in said bubbling dense bed,adding a stripping gas to said hot stripper, and stripping additionalcracked products from said stripped catalyst to produce hot strippedcatalyst which is charged to said catalyst regeneration zone and hotstripper vapor which is removed as a product and further characterizedin that the bubbling dense bed regenerator discharges a flue gas andentrained catalyst stream into a dilute phase region above the bubblingdense bed, and at least one stage of cyclone separation is used torecover catalyst from flue gas, and wherein said hot stripper is alsoheated by addition of hot regenerated catalyst recovered from saidcyclone separator.
 2. The process of claim 1 wherein said hot strippingmeans comprises multiple means for stripping gas addition and multiplemeans for removal of stripping gas and stripped hydrocarbons.
 3. Theprocess of claim 2 wherein said hot stripper stripping gas and strippedproduct removal means communicates with a vapor space containing saiddisengaging zone used to separate cracked products from catalyst exitingthe riser reactor.
 4. A fluidized catalytic cracking process wherein aheavy hydrocarbon feed comprising hydrocarbons having a boiling pointabove about 650° F. is catalytically cracked to lighter productscomprising the steps of:catalytically cracking said feed in a catalyticcracking zone operating at catalytic cracking conditions by mixing, inthe base of a riser reactor, a heavy crackable feed with a source of hotregenerated catalytic cracking catalyst withdrawn from a catalystregenerator, and cracking said feed in said riser reactor to producecatalytically cracked products and spent catalyst which are dischargedfrom the top of the riser into a catalyst disengaging zone whereincracked products are separated from spent catalyst; separating crackedproducts from spent catalyst in said catalyst disengaging zone toproduce a cracked product vapor phase which is recovered as a productand a spent catalyst phase which is discharged from said disengagingzone into a primary catalyst stripper contiguous with and beneath saiddisengaging zone; stripping said spent catalyst with stripping gas insaid primary stripping zone to produce a stripper vapor comprisingcracked products and stripping gas which is removed from said strippingzone as a product and a stripped catalyst phase comprising strippedcatalyst having a temperature is discharged into a vertical standpipebeneath said primary stripping zone; hot stripping catalyst dischargedfrom said primary stripper standpipe in a hot catalyst stripping zonebelow said primary stripping zone by heating spent catalyst in saidstandpipe by indirect, countercurrent heat exchange with a dilute phasemixture of hot regenerated catalyst and flue gas and discharging spentcatalyst into a hot stripping zone which is heated by indirect heatexchange with a fast fluidized or turbulent fluidized bed of catalystand regeneration gas and stripping the heated catalyst in said hotstripping zone with a stripping gas to produce hot stripped catalystwhich is discharged from said hot stripping zone and recovering a hotstripper effluent vapor stream comprising stripping gas and strippedlighter products via a closed conduit means which passes through saidprimary stripper; regenerating hot stripped catalyst discharged fromsaid hot stripping zone in a regeneration zone comprising a fastfluidized or turbulent fluidized bed of catalyst maintained in a closedcoke combustor vessel, which is at least partially immersed in saidbubbling dense bed, said coke combustor vessel having a base region witha cross sectional area and an upper region of reduced cross sectionalarea relative to said base region, by adding an oxygen containingregeneration gas to said coke combustor vessel in an amount sufficientto provide a superficial vapor velocity which maintains a majority ofthe catalyst therein in a state of turbulent or fast fluidization;transferring said catalyst from said base region of said coke combustorto said upper region of said coke combustor having a reduced crosssectional area, whereby increasing the superficial vapor velocity andcausing dilute phase catalyst transport in said upper region;discharging at least partially regenerated catalyst from said upperregion into a dilute phase transport riser connective with said cokecombustor vessel and at least partially enclosing said primary strippercatalyst standpipe, said dilute phase transport riser extending intosaid dilute phase region within said regenerator vessel containing saidbubbling fluidized bed; and discharging regenerated catalyst from saiddilute phase transport riser and collecting said regenerated catalyst insaid bubbling fluidized bed surrounding said coke combustor and addingto said bubbling fluidized bed fluffing air in an amount sufficient tomaintain a bubbling fluidized bed and to produce a dilute phase regionabove said bubbling bed comprising fluffing air and regeneratedcatalyst.
 5. The process of claim 4 wherein the catalyst in said hotstripping means is also heated by direct contact heat exchange with hotregenerated catalyst.
 6. The process of claim 5 wherein the amount ofhot regenerated catalyst added is 5 to 250 wt. % of the spent catalyst.7. The process of claim 5 wherein the amount of hot regenerated catalystadded is 10 to 100 wt. % of the spent catalyst.
 8. The process of claim5 wherein at least one stage of cyclone separation is used to recovercatalyst from fluffing air and regenerated catalyst in the dilute phaseregion above the bubbling dense bed, and wherein said hot stripper isalso heated by addition of hot regenerated catalyst recovered via saidcyclone separator.
 9. The process of claim 5 wherein the bubbling densebed regenerator maintains a fluidized bed of regenerated catalyst havingan elevation and said hot stripper comprises a lower, direct contactheat exchange region at an elevation below the elevation of the bubblingbed and wherein catalyst in said hot stripper is heated by transferringhot regenerated catalyst via a flow control means from said bubblingdense bed to said direct contact heat exchange zone.
 10. The process ofclaim 9 wherein said flow control means comprises a catalyst flow lineconnective with said bubbling dense bed and with said direct contactheat exchange region of said hot stripper, and comprising a slide valve.11. The process of claim 4 wherein said hot stripping means comprisesmultiple means at a plurality of elevations for stripping gas additionand multiple means at a plurality of elevations for removal of strippinggas and stripped hydrocarbons.
 12. The process of claim 11 wherein saidhot stripper stripping gas and stripped product removal meanscommunicates with a vapor space containing the disengaging zone used toseparate cracked products from catalyst exiting the riser reactor.
 13. Afluidized catalytic cracking process wherein a heavy hydrocarbon feedcomprising hydrocarbons having a boiling point above above 650° F. iscatalytically cracked to lighter products comprising the stepsof:catalytically cracking said feed in a catalytic cracking zoneoperating at catalytic cracking conditions by mixing, in the base of ariser reactor, a heavy crackable feed with a source of hot regeneratedcatalytic cracking catalyst withdrawn from a catalyst regenerator, andcracking said feed in said riser reactor to produce catalyticallycracked products and spent catalyst which are discharged from the top ofthe riser into a catalyst disengaging zone wherein cracked products areseparated from spent catalyst; separating cracked products from spentcatalyst in said catalyst disengaging zone to produce a cracked productvapor phase which is recovered as a product and a spent catalyst phasewhich is discharged from said disengaging zone into a catalyst strippercontiguous with and beneath a said disengaging zone; steam strippingsaid spent catalyst with stripping steam in said stripping zone toproduce a stripper vapor comprising cracked products and stripping steamwhich is removed from said stripping zone as a product and a strippedcatalyst phase comprising stripped catalyst having a temperature isdischarged into a vertical standpipe beneath said stripping zone;discharging stripped catalyst from said standpipe into a catalystregeneration zone contiguous with and beneath said stripping zone;regenerating said stripped catalyst in a regeneration zone comprises asingle dense phase bubbling fluidized bed of catalyst to which an oxygencontaining regeneration gas is added and from which hot regeneratedcatalyst, having a regenerated catalyst temperature above said strippedcatalyst temperature, is withdrawn and recycled to said riser reactor,characterized by: discharging said stripped catalyst from said catalyststandpipe into a hot stripper means which is at least partially immersedin said bubbling dense bed, and heating said stripped catalyst in saidhot stripper means by indirect heat exchange with hot regeneratedcatalyst in said bubbling dense bed, adding a stripping gas to said hotstripper, and stripping additional cracked products from said strippedcatalyst to produce hot stripped catalyst which is charged to saidcatalyst regeneration zone and hot stripper vapor which is removed as aproduct and further characterized in that said hot stripper meanscomprises a plurality of tubes having inlets in a top portion thereoffor spent catalyst and permitting discharge of stripper vapor, andoutlets in a base portion thereof for discharge of stripped catalystinto said catalyst regeneration zone.
 14. The process of claim 13wherein said hot tubes are radially disposed and wherein a ring shapedbaffle is provided above the tube inlets to provide for more orderlyaddition of spent catalyst to the tubes and for more orderly withdrawalof stripper vapor.